A growing number of atomic force microscope (AFM) applications make use of metal-coated probes. Probe metallization can cause adverse side-effects and disadvantages such as stress-induced cantilever bending, thermal expansion mismatch, increased tip radius and limited device lifetime due to coating wear.
Since the inception of the atomic force microscope (AFM) [1], its versatility and usefulness as a characterization, measurement and fabrication tool has expanded beyond simple topographic imaging. Currently AFM probes are typically made from Si [2], SiO2 and Si3N4 [3]. These silicon-based materials are so ubiquitously used since they are elastically stiff, very hard and easy to process. Unfortunately these materials possess low electrical conductivity and poor optical reflectivity. An increasing number of applications make use of optically reflective cantilevers and electrically or thermally conducting tips, which is typically accomplished by coating the AFM probe with a metal layer. Despite providing the desired optical, thermal or electrical properties, metal coatings produce adverse side-effects.
The extremely high spatial resolution of the AFM is what makes it such a unique and useful tool. Important technological applications that make use of the high spatial resolution of a conductive AFM tip are conductive atomic force microscopy (C-AFM) [4], thermomechanical data storage [5-7], electrochemical nanolithography by local metal etching [8], local anodic oxidation [9] and ferroelectric data storage [10], to name just a few. A fundamental problem caused by metallization of the AFM tip is increased tip radius, resulting in a loss of spatial resolution. It is straightforward to show that the tip radius of a conformally coated parabolic tip is the sum of the coating thickness and tip radius prior to coating. Therefore, tip radius is significantly restricted by the thickness of the metallization layer, which is typically made to a minimum thickness of 15-20 nm [8] simply to ensure film continuity. This problem is further compounded by wear of the metal coating.
It has recently been shown that, after acquiring just a few C-AFM maps, wear of the tip coating produces significant artifacts which are often quite difficult to detect [4]. Wear of metal-coated AFM tips is also particularly important in SPM-based recording technologies, such as ferroelectric data storage [10, 11], which promises ultrahigh areal data density.
Increasing the thickness of the deposited metal layer is not a viable solution to increasing probe lifetime, as the tip radius becomes significantly larger and stresses in the metal layer can induce significant bending of the cantilever, to the extent that alignment in the AFM system becomes impossible [12].
As shown by Birkelund et al. [12] the use of all-metal probes for AFM nanolithography resulted in a tenfold increase in lifetime compared to conventional titanium-coated silicon nitride cantilevers. This large enhancement in device lifetime was a result of maintaining conductivity despite continuous wear of the tip. These probes were fabricated by a combination of silicon micromachining and electroforming, resulting in nickel probes with a gold coating. Due to the bilayer nature of these probes they are still susceptible to thermal mismatch bending [13]. Other authors have fabricated single-layer all-metal AFM cantilevers [14], but are limited to metals that can be electroplated, with nickel often being the material of choice. Moreover, these cantilevers are often made to be very stiff (low force sensitivity, thickness >4 μm) to avoid excessive cantilever bending due to residual stress gradients that can develop during film growth. Chand et al. reported a process for the fabrication of high resonant frequency and force sensitivity bilayer Au/Ti cantilevers with an integrated silicon tip [15]. The combination of high resonant frequency and force sensitivity was achieved by reducing the cantilever dimensions (13-40 μm long and 100-160 nm thick). These cantilevers were unusably bent upon initial release due to residual stress gradients caused by competitive grain growth, but the devices were straightened using rapid thermal annealing. Unfortunately this process only produced a 60% device yield.
All-metal AFM probes consisting of a single material will have tip radii limited by the fabrication procedure, possess superior device lifetime (independent of tip radii) and be immune to thermal expansion mismatch bending. It has been shown that uncurled metal cantilevers can be fabricated from thin film metallic glasses [16, 17]. The uncurled nature of the cantilevers was attributed to the lack of grains and consequent differential stress induced by grain size gradients throughout the film thickness. Due to the specific thermodynamic properties of the alloys used, large solute content was needed in order to achieve the desired amorphous microstructure, which resulted in an undesirable loss in electrical conductivity [16, 17]. Moreover, chemical etching of these films proved difficult due to the high solute content, limiting fabrication to small thicknesses using a liftoff process.